In the above illustration, the red area is the approximate orientation of the Navajo
volcanic field, and the letters are used to identify two of the more well-known diatremes,
Shiprock(SR) and Agathla(AG). There are numerous other volcanic features throughout this region.
Although Shiprock is believed to have been created as much as 1,000 meters below the Earth's
surface, millions of years of differential erosion have uncovered the main volcanic neck, or
plug, as well as associated smaller plugs and six dikes radiating out from the center. The
following diagram shows the layout of these features from above.

Agathla, located on the first diagram as AG, was formed in a similar fashion as Shiprock.
Both structures are believed to be the result of "maar" eruptions, where an explosive event
produces a round crater at low relief, usually filling with water. Agathla can be seen near
the entrance to Monument Valley in northern Arizona. The following figure illustrates how
Agathla and other similar structures in the area were created, as well as how erosion plays a
part in their appearances today.

The following photo demonstrates how Agathla and Shiprock have
similar physical characteristics. The two monoliths are found approximately 120 km apart.

Shiprock is comprised of a form of magma known as volcanic breccia, pronounced
"BRET-cha". This is a material composed of smaller rock fragments cemented together, similar
to conglomerate but with sharper, more irregular pieces. The jagged fragments hint at the
explosive nature of the eruption that created Shiprock. In fact there is an abundance of
boulders consisting of tuff-breccia near the base of Shiprock, displaying components of varying
sizes and colors. This is characteristic of a violent volcanic event, welding ash and rock
together.

The magma of Shiprock is composed of an uncommon, highly potassic material
referred to as "minette", or an orthoclase biotite lamprophyre, which is believed to result from
extremely gradual melting of mantle rock. Minette consists of alkali feldspar, biotite or
phlogopite, and diopside(pyroxene). This minette also contains crystals of augite and olivine as
well as biotite mica flecks. Lacking plagioclase, it is dark brown to black with a density and
texture similar to basalt. As the magma cooled, it formed vertical cracks producing an irregular
columnar jointing. It is believed that the magma which fed Shiprock may have
traveled as much as 35km from the Earth's mantle through igneous and metamorphic rock, including
over 3000 meters of old sediments. Perhaps the release of pressurized gas and the interaction
with sub-surface water produced a violent eruption called a "mantle blowout".
A striking feature of Shiprock is the presence of dikes radiating out
from the main formation. These formed as magma filled cracks in the ground during a period of
eruption. The same process of erosion that uncovered the neck of Shiprock has also revealed
these dikes as walls of lava. Scientists believe the tuff-breccia was first on the scene during
the initial explosion, followed by the gas-poor minette lava as it slowly filled
the dikes as well as the cracks in the neck. We can assume the minette lacked appreciable gas
as the final product is void of vesicles, or air bubbles. Following are two photos of these
remarkable examples of dikes.

What is the origin of the forces behind the formation of volcanic features as Shiprock,
Agathla and so many others in the Four Corners region? It appears that around 30 million years
ago, the approximate time of the Shiprock eruption, the land that is now New Mexico began to
stretch apart, forming the Rio Grande Rift. Running roughly north to south from southern
Colorado through New Mexico, this geological feature bisects the eastern edge of the Colorado
Plateau. Previously, this region of the North American continent was actually believed to be a
subduction zone. The transformation of a converging plate boundary to one of strike-slip, or
transform faulting, appears to coincide with the beginnings of the rift valley.

There is some discrepancy between scientists as to whether magma rising up towards
the Earth's surface leads to spreading apart of the crust, or if the thinning surface material
"invites" the magma to enter through the path of least resistance. Either way, the result was
large amounts of magma near the surface. As magma nears the crust, decreased pressure allows gas
to escape the mixture, often explosively.

Throughout New Mexico are signs of
stress within the Earth's crust. The Rio Grande Valley, to the southeast of Shiprock, is an
example of a horst and graben formation. The Sandia Mountains on the east side of Albuquerque
are the horst, lifted up along a fault line tens of thousands of kilometers above the city while
the valley through which the Rio Grande River flows is the graben, having descended well below
the valley floor. There is still some volcanic activity in the region, evidenced by geothermal
hot springs and minor tremors.

Most people consider the formations of the Navajo
Volcanic Field extinct, as they have not been active for millions of years (the formations, not the
people). However, the existence of geothermal activity farther south as well as lava flows in
the central part of New Mexico and Arizona that are only thousands of years old makes one
wonder.